This invention relates to the prevention of the decomposition of salts of anthraquinonedisulfonic acid, particularly when the salts are in an aqueous alkaline solution. Such solutions are generally provided in systems for the removal of gaseous sulfur-containing compounds, such as hydrogen sulfide, from various industrial, gaseous mixtures. The invention has special applicability to scrubbing solutions, containing undesired compounds, especially alkali sulfates, formed during the course of the scrubbing step, requiring that a partial stream of the solution be subjected to combustion under reducing conditions so as to remove the sulfates.
In the scrubbing step of gas scrubbing systems as outlined above, the salts of anthraquinonedisulfonic acid contained in the aqueous alkaline solution, the scrubbing liquid, serve, together with vanadium, as oxidizing agents which themselves are reduced during the course of the chemical reactions. To restore their effectiveness, a reoxidation is subsequently conducted with oxygen or an oxygen-containing gas. The sulfur contained in the compounds absorbed by the scrubbing liquid is separated and recovered in elemental form, and the regenerated scrubbing liquid is recycled into the scrubbing stage.
Certain secondary reactions occur during the course of the scrubbing step, especially due to the introduction of oxygen, resulting in the formation of, in part, undesirable compounds. Among these are, above all, alkali sulfates, the concentration of which in the circulating scrubbing liquid must not rise unchecked. In order to remove the thus-formed by-products, a correspondingly dimensioned proportion of the scrubbing liquid is therefore branched off from the main stream and subjected to a special treatment. This treatment resides in a combustion taking place under reducing conditions, during which the by-products are reacted to alkali carbonates, alkali hydrogen carbonates, as well as hydrogen sulfides and/or sulfides. The liquid combustion product stemming from the combustion and containing these compounds can then be recycled into the main stream of the scrubbing liquid, since the interfacing by-products have been converted.
Besides the alkali sulfates, alkali thiosulfates can also be formed during the course of the scrubbing process, which then are likewise extensively decomposed during combustion in correspondence with their proportion in the scrubbing liquid branched off from the main stream. The sulfates and thiosulfates can be formed in greatly varying quantitative ratios, depending on the specific process conditions. Thus, it can also happen that thiosulfate formation occurs either not at all or only to a very minor extent.
However, it has been found that the last-mentioned aspect is of significance in connection with maintaining the anthraquinonedisulfonate concentration in the circulating scrubbing liquid. For the anthraquinonedisulfonates are gradually decomposed, for reasons not fully understood as yet, so that these relatively expensive compounds must be replenished continuously or at regular intervals. However, if thiosulfates are present in the scrubbing liquid in corresponding amounts, then the extent of anthraquinonedisulfonic decomposition recedes markedly and, in certain cases, can even be stopped entirely.
However, it has not always been possible heretofore in all cases to establish and/or maintain an adequate thiosulfate concentration in the circulating scrubbing liquid, especially if, during the course of the scrubbing operation, the amount of thus-formed thiosulfate was nil or only very minor.